Sticky exhaust valves

[Spetz]

Hi everyone,

I have a 3.8 Mitsubishi 6G75 on an engine stand preparing it for use.

Using a leakdown tester to keep the valves up while replacing valve stem seals.

I noticed a couple of exhaust valves are dirty from carbon deposits (presumably oil has been leaking through the seals), and only seat in one direction.

Is this an issue?
Will this correct itself with normal operation after the valve stem seals are replaced?
Is there a way to correct this without removing the heads?



The engine is on a stand so accessibility is good.

 

[LazyDog]

Why not clean them, lots of You Tube videos on how to do them.

 

[Spetz]

Why not clean them, lots of You Tube videos on how to do them.

I am trying to clean them, and will continue to clean more.
However with a leakdown tester, air is escaping from them still.
So I am not sure if this means heads off to get it fixed

 

[PontiacHO]

If leakdown test shows valves not seating then they (and seats) most likely have to be refaced. Now is the time to do that with engine on the stand.

 

[Donald]

What is the history of the engine? Sounds like the head needs a valve job at a machine shop. As others have said, now is the time.

You need to start with pulling the head. Maybe it's just carbon deposits that can be removed with a wire brush. Maybe burned valve.

 

[dumkid]

Try tapping on the valve stem with a small hammer once the springs are back on the valve. If the valve quits leaking once you tap on it then it's sticky, not burned.

I have a 3.5 6g74 with sticky intake valves on one cylinder. If you tap on the valve stems they stop leaking. If you oil them (tcw3 2 stroke oil in the gas tank) they stop leaking from a little while. If it's warm outside or the engine is warm they don't stick. If I run b12 chemtool they seem to stick worse when the outside temp is really cold, but less when it's warmer outside. I'm hoping the b12 will clean up the valve guides and help the cold start miss to dissapear, but I've only been running it for about 100 miles. The valves only stick for the first 30 seconds after you start the car, and that's just enough to set a misfire code. My car did sit for 4 years and was overheated enough to blow a head gasket in the distant past. That could have something to do with my problem.

What are you putting this engine in? My 6g74 is in a 01 Montero

 

[Spetz]

Try tapping on the valve stem with a small hammer once the springs are back on the valve. If the valve quits leaking once you tap on it then it's sticky, not burned.

I have a 3.5 6g74 with sticky intake valves on one cylinder. If you tap on the valve stems they stop leaking. If you oil them (tcw3 2 stroke oil in the gas tank) they stop leaking from a little while. If it's warm outside or the engine is warm they don't stick. If I run b12 chemtool they seem to stick worse when the outside temp is really cold, but less when it's warmer outside. I'm hoping the b12 will clean up the valve guides and help the cold start miss to dissapear, but I've only been running it for about 100 miles. The valves only stick for the first 30 seconds after you start the car, and that's just enough to set a misfire code. My car did sit for 4 years and was overheated enough to blow a head gasket in the distant past. That could have something to do with my problem.

What are you putting this engine in? My 6g74 is in a 01 Montero

I will clean them and see if I can get them to seal when doing a leakdown test.

The engine is going into a 1999 widebody Montero. Here in Australia it is called a Pajero

 

[dumkid]

I will clean them and see if I can get them to seal when doing a leakdown test.

The engine is going into a 1999 widebody Montero. Here in Australia it is called a Pajero

2 door or 4 door? Seems like a cool build. There is an Australian Pajero forum, if you aren't already in it.

 

[Spetz]

2 door or 4 door? Seems like a cool build. There is an Australian Pajero forum, if you aren't already in it.

4 door.
And yes I am on it.
But I find this forum is really good for general mechanical expertise as so many people here have so much relevant experience.

 

[tiger862]

Why not pull the heads while on the stand and remove valve from the head and hand lap the valve and seat?

 

[Spetz]

Why not pull the heads while on the stand and remove valve from the head and hand lap the valve and seat?

I think I will try and do that.
Initially though I may try cleaning the valves from the port.
If that doesn't work then I will pull the head(s) off. I am not sure if the other head (of the V6) has the same issue or not, so might get away with just 1 head.

I was trying to avoid that purely on the cost of the head gasket and head bolts, as OEM parts will end up being $300+ here.

Plus, if the heads are off, I fall into the trap of "might as well do rings and bearings" since I can remove the pistons and rods

 

[3800Series]

This is somewhat common with junkyard motors which have sat for awhile, I wouldn't be too concerned ATM.

Spin the crank until it about as sealed as it will get, spray some cleaner down the exhaust port and try clean it with a little brush. If you are able too it wouldn't be a bad idea to remove the plug and pressurize that cylinder while doing so.

Spin the crank, open the valves spray some mkre cleaner, pressurize, etc.

It can take a few rounds

Give it a few good attempts ideally it will slowly build pressure.

 

[dumkid]

sorry to resurrect this but putting a can of b12 truck and SUV in my Montero fixed the cold start miss in a few hundred miles. I've tried seafoam, tcw3 (which worked a little bit) mmo, and Lucas fuel injector cleaner, and I've taken the injectors out and tested them to see if they had a good spray pattern. I did run b12 one of the first tanks after I bought the car, and it didn't do anything back then so i didnt know if it would do anything now.

 

[MattPersman]

sorry to resurrect this but putting a can of b12 truck and SUV in my Montero fixed the cold start miss in a few hundred miles. I've tried seafoam, tcw3 (which worked a little bit) mmo, and Lucas fuel injector cleaner, and I've taken the injectors out and tested them to see if they had a good spray pattern. I did run b12 one of the first tanks after I bought the car, and it didn't do anything back then so i didnt know if it would do anything now.

Good to hear your outcome was positive after all